**2.3.3 Acetogenesis**

154 Biogas

Anaerobic digestion is a biological process, which is used for the treatment and valorisation of organic waste. Generally It goes through the four steps, as mentionned above, and which are hydrolysis, acidogenesis, acetogenesis and methanogenesis. In the case of co-digestion biodegradable solid waste is added at the head of the process. A preliminary stage of disintegration of the substrate, which is in general a nonbiological step for the transformation of the complex polysaccharide, lipids and proteins, is considered (Thiele,

The hydrolysis is an extracellular process in which complex particulate organic substances (proteins, polysaccharides, lipids, cellulose… etc) are broken up into simple soluble compounds (acid amino, simple, acid sugars fatty, glycérol… etc). It is a significant stage before the process of fermentation, because the fermentative bacteria cannot absorb complex organic polymers directly in their cells. The hydrolytic enzymes include the cellulase, the cellobiase, the xylanase and amylase for the decomposition of sugar polysaccharides, the protease for the degradation of the protein in amino acids, and lipase for the degradation of the glycerol lipids and the fatty acids with long chain (LCFA) (Batstone & al., 2002 and

The acidogenic step consists of a degradation of produced components from the hydrolysis step, by the action of acidogenic and fermentative bacteria. It leads to the formation of a mixture of: organic acids, volatile fatty acid (VFA), alcohols, hydrogen, carbon dioxide,

Examples of the various products of the fermentation of glucose are shown in the following

6 12 6 2 3 2 2

*C H O H O CH COOH CO H*

2 2 24 3 4 2 22

+ → ++

→ + +

6 12 6 322 2 2

Table 1. Exemples de la fermentation du glucose (Dolfing, 1988; Angelidaki & Ellegaard,

The dominant route depends on several factors such as the concentration in substrate, pH and hydrogen concentration (Balk et al., 2002). Under a very high organic load, the lactic acid production becomes significant (Mattiasson, 2004). With low pH (lower than 5) the production of ethanol is high, whereas with lower pH (lower than 4) there is a strong

*C H O CH CH CH COOH CO H*

2 2

6 12 6 3 2 2

→ +

2

*C H O CH CHOHCOOH C H O CH CH OH CO*

6 12 6 3

production of the volatile fatty acids (VFA) (Ren & al., 1997).

→

6 12 6 3 2 3 2 2

*C H O CH CH COOH CH COOH CO H O*

→ + ++

2 2

**2.3 Anaerobic digestion steps** 

1991).

**2.3.1 Hydrolysis** 

Kaseng & al., 1992).

**2.3.2 Acidogenesis** 

ammonium, etc.

Products Reactions

Table 1:

Acetate

Propionate + Acetate Butyrate Lactate Ethanol

2002 ; Rodriguez, 2006)

The acetogenic step allows the transformation of the acids, resulting from acidonenic step to acetate, and carbon dioxide, by the action of the acetogenic bacteria. This operation is carried out by different types of bacteria.
